vrrp: fix preempt and state BACKUP when prio 255

This makes it so that keepalived will respect various settings that
should prevent it from assuming the MASTER role for a vrrp_instance
unconditionally and immediately, even if the priority of the
vrrp_instance in question is set to 255 (VRRP_PRIO_OWNER). These
settings include:

  state BACKUP
  preempt_delay <N>
  nopreempt